This invention relates to a tubular reactor, and for processes of conducting liquid/liquid multiphase reactions, particularly nitrations of aromatic compounds, in tubular reactors.
Reactions between immiscible or only slightly miscible liquids are commonly performed. Typical such reactions include an aqueous phase that is reacted with an immiscible organic phase. Because the desired chemical reactions usually occur mainly at the interface of the liquid phases, an important factor in obtaining a complete reaction or a commercially acceptable rate of reaction is to intensely mix the phases. There are various ways of accomplishing this. A common way is to conduct the reaction with mechanical mixing, such as by using an agitator blade. Cascade reactors are also known. Apparatus of these types have various shortcomings. Moving parts tend to wear out and need maintenance or replacement. Usually the apparatus is relatively expensive. Often, back-mixing occurs, leading to the formation of undesired byproducts or in some cases, over-reaction of the raw materials.
The problems associated with reactions between immiscible liquids are illustrated well by the nitration of aromatic compounds. Two commercially important nitrated aromatic compounds are mononitrobenzene (MNB) and dinitrotoluene, which are prepared by nitrating benzene and toluene, respectively. MNB is a common solvent and can be converted to another commercially important compound, aniline. Similarly, nitrated toluene products such as dinitrotoluene are used to make derivatives such as toluene diamine, which can be further converted to toluene diisocyanate, an important raw material for making polyurethane polymers.
Aromatic ring nitration reactions are ordinarily conducted by mixing the aromatic compound with nitric acid in the presence of sulfuric acid. An adiabatic process for producing mononitrobenzene is described in U.S. Pat. No. 2,256,999 to Castner. In Castner's process, as in all similar benzene nitration processes, the acids form a phase that is immiscible with the aromatic compound. Consequently, Castner describes using a series of agitated tanks for conducting the reaction in order to obtain a commercially acceptable reaction rate. However, the Castner process suffers from several difficulties, primarily low yields and the formation of high levels of nitrophenolic impurities. In addition, the Castner process forms undesirably high levels of overnitrated products, primarily dinitrobenzene.
The reliance on high shear mixing to deal with immiscible raw materials is reflected in other nitration processes as well. In U.S. Pat. Nos. 4,021,498 and 4,091,042, Alexanderson et al. describe using a “vigorously agitated” tubular reactor to conduct the reaction. This alone was not sufficient to satisfactorily produce the desired product, however. Consequently, Alexanderson et al. require careful selection of the proportions of starting materials in order to reduce the level of impurities in the product. This general approach to reducing impurities was continued in U.S. Pat. No. 5,313,009 to Guenkel et al., in which impurity formation is said to be reduced using a specially designed mixer, which produces extremely fine benzene bubbles in the acid phase, followed by a tubular reactor that may include additional static mixing elements. Like Alexanderson et al., Guenkel et al. found that very specific proportions of starting materials were necessary in order to obtain a product with low levels of impurities.
Other references are similar. In U.S. Pat. No. 3,431,312 to Engelbert et al., nitration is performed in a series of cascade reactors, all of which are equipped with mixers or stirrers. In U.S. Pat. No. 4,973,770 to Evans, the reaction is performed by forming a turbulent jet of nitric and sulfuric acid to produce droplets of mixed acid having a size of from less than 1 μm to about 10 μm in diameter and contacting the acid droplets with the nitratable organic compound. In U.S. Pat. No. 5,963,878, a pipe nitrator discharges into a stirred tank type reactor.
In the process described in U.S. Pat. No. 5,763,687 to Morisaki, the reaction is conducted in a tube or pipe reactor equipped with a number of specially designed static mixing elements.
Thus, nitrations of aromatic compounds typify many of the problems that attend multiphase liquid/liquid reactions. On the one hand, for economic reasons it is necessary to obtain an acceptable reaction rate, and this is usually facilitated by increasing the contact between the phases. On the other hand, over-contacting the phases can cause impurities, particularly nitrophenols and cresols to form. Similarly, back-mixing or over-contacting the phases may cause even the desired reaction to go too far. With nitration reactions, this is seen in the production of over-nitrated products such as dinitrobenzene (in MNB production). The formation of impurities in this manner reduces yield, thereby reducing the overall economic efficiency of the process.
Thus, it would be desirable to provide an apparatus with which multiphase liquid/liquid reactions can be conducted, which provides good control of the reaction and efficient mixing of the phases. It would also be desirable to provide a process for conducting multiphase liquid/liquid reactions efficiently, with good yields and low levels of impurities and byproducts being formed. In particular, it would be desirable to provide a method of nitrating aromatic compounds with good yields, low levels of nitrophenolic impurities and low levels of undesired by-products, using relatively inexpensive equipment.